Geomagnetic navigation

Sea turtles carry out long-distance migrations between their feeding grounds and their breeding grounds. For example, some green sea turtles feeding off the coast of Brazil migrate over 2000 kilometers to Ascension Island in the middle of the Atlantic. But how do the turtles navigate the oceans? One hypothesis is that they use Earth’s magnetic field for navigation. This was once tested by attaching powerful static magnets to turtles and seeing if it would impede the turtles’ navigational skills. Results from such studies were the first field evidence for the involvement of geomagnetic cues in sea-turtle navigation.

In a new research published recently in the scientific journal Current Biology, researchers from the University of North Carolina were interested in knowing if marine turtles could imprint on the geomagnetic signature of their natal beach and use that information to return to it year after year. Their starting hypothesis was that if turtles imprint on their natal beach using a geomagnetic signature, the turtles would nest at slightly different locations year after year since Earth’s magnetic field shifts slightly over time. To test this they mapped out 19 years of loggerhead turtle nesting data along the Atlantic coast of Florida. They observed that nesting locations would vary from year to year with some beaches being more popular than others depending on the year. They then examined how Earth’s magnetic field changed at their study site over the same period of time and found that there was indeed a strong association between the spatial distribution of turtle nests and subtle changes in Earth’s magnetic field. So it would seem that turtles are able to imprint on their natal beach’s unique geomagnetic signature and use this information to return there to nest!

James Gould, a professor in Ecology and Evolutionary Biology at Princeton University, reports in Current Biology that this is an “extremely clever analysis” in which the authors of the study took “initially unpromising data” and looked at it in an “entirely new way.” But what is the reason for a turtle to return to the beach where she was born in order to nest? J. Roger Brothers, one of the authors of the paper, says: “The only way a female turtle can be sure that she is nesting in a place favourable for egg development is to nest on the same beach where she hatched. The logic of sea turtles seems to be that ‘If it worked for me, it should work for my offspring.'”

Long-distance migrations

Most sea turtles come back to nest on the beach where they were born. How they do this, is still a mystery.

In their breeding years turtles will migrate from their feeding grounds to their nesting grounds. Before modern technology was available, scientists could only guess how far or where the turtles would go. Nowadays, GPS tags allow us to study the migrating behaviour of sea turtles.

The most common practice is to tag a nesting female turtle by placing a satellite tag on her shell. Out in the ocean, the GPS calculates the geographic position of the turtle every time she surfaces to breath. This information is relayed to the researcher’s computer via satellite. The turtle can be tracked for as long as the tag works, which can be weeks, months, and sometimes even years.

A green sea turtle returning to the sea with a satellite tag on her carapace (photo couretesy of Nicole Esteban)

A green sea turtle returning to the sea with a satellite tag on her carapace
(photo courtesy of Nicole Esteban)

Thanks to satellite telemetry studies, we now know where nesting turtles have their feeding grounds. For example, we now know that some of the turtles nesting on Ascension Island migrate to their feeding grounds in Brazil. This migration is quite a feat, considering that the trip from Brazil to Ascension Island is over 2000 kilometres and the turtles’ destination is a remote island less than twenty kilometres across.

Science has yet to find out how turtles navigate such vast distances. One hypothesis is that they use Earth’s magnetic field for navigation. In fact, a turtle’s ability to navigate the oceans of the world is one that amazed Charles Darwin himself, as shown in his Letter to Nature in 1873:

“Even if we grant to animals a sense of the points of the compass, of which there is no evidence, how can we account, for instance, for the turtles which formerly congregated in multitudes, only at one season of the year, on the shores of the Isle of Ascension, finding their way to that speck of land in the midst of the great Atlantic Ocean?”

Recently, the longest ever published migration for an adult cheloniid (a hard-shelled marine turtle) was recorded: a green turtle swam almost 4000 kilometres from its breeding grounds in Diego Garcia to its feeding grounds off the coast of Somalia!